The present invention relates to a thermal transfer image-receiving sheet comprising a receptive layer provided on a substrate sheet, and more particularly to a thermal transfer image-receiving sheet of a type such that a metal source (a metal ion-containing compound) is contained in the receptive layer and, upon the transfer of a dye, which can be chelated, from a thermal transfer sheet onto the receptive layer, the dye is chelated and is fixed onto the receptive layer, which thermal transfer image-receiving sheet, when a protective layer is transferred on the receptive layer with the image formed thereon, the receptive layer has excellent adhesion to the protective layer.
In the formation of an image in response to image information by means of thermal printing means, such as a thermal head or a laser, a method has been proposed wherein a thermal transfer sheet provided with a sublimable dye layer, which is transferable upon heating, is used in combination with a thermal transfer image-receiving sheet and the dye is transferred onto the thermal transfer image-receiving sheet while controlling the sublimable dye to form a gradational photograph-like image. This method is advantageous, for example, in that images with continuous gradation can be provided by simple processing from digital image data on a digital camera or a personal computer or image data through a network and television signals and, in this case, the apparatus used is not complicate.
In this type of thermal transfer recording, the sublimable dye used in the thermal transfer sheet plays an important role. Conventional sublimable dyes, however, have a drawback that the formed images have unsatisfactory fastness properties, that is, unsatisfactory lightfastness and fixation. In order to reduce this problem, Japanese Patent Laid-Open Nos. 78893/1984, 109394/1984, and 2398/1985 disclose an image forming method wherein a heat diffusive colorant (dye), which can be chelated, is used to form an image of a chelated colorant (a chelate dye) on a thermal transfer image-receiving sheet.
The method for forming an image of a chelated dye is effective for improving the heat resistance and lightfastness of images and the dye fixation. In this method, however, after printing, the dye remaining unreacted is present around the surface of the receptive layer, and thus results in unsatisfactory fastness properties of transferred images. Specifically, when the formed image comes into contact with a finger or when the formed image is continuously in contact with a plasticizer-containing sheet, for example, dropouts occur, making it difficult to maintain the image quality. In order to reduce this problem, an attempt has been made to thermally transfer a protective layer onto an image from a protective layer transfer sheet having a thermally transferable protective layer. This, however, has posed a problem that the metal source (metal ion-containing compound) present in the receptive layer inhibits the adhesion between the protective layer and the receptive layer and the protective layer is easily separated from the receptive layer.
Accordingly, it is an object of the present invention to provide a thermal transfer image-receiving sheet which, when a protective layer is formed by thermal transfer on an image formed of a chelated dye, can provide excellent adhesion of the protective layer onto the image and can realize good image quality.
According to one aspect of the present invention, there is provided a thermal transfer image-receiving sheet comprising: a substrate sheet; and a receptive layer provided on the substrate sheet, said receptive layer comprising a metal source, a protective layer bonding/holding agent, and a binder resin.
The protective layer bonding/holding agent is preferably a surfactant having a polyoxyalkylene group. The surfactant is preferably a fluorosurfactant or a polyether-modified silicone.
The fluorosurfactant preferably has a straight-chain polyoxyalkylene group, and the polyether-modified silicone is preferably a silicone modified by providing a polyoxyalkylene group on its side chain.
Preferably, the receptive layer contains a release agent.
According to a preferred embodiment of the present invention, the content of the protective layer bonding/holding agent is in the range of 0.25 to 7.5% by mass based on the solid content of the whole receptive layer. More preferably, the protective layer bonding/holding agent comprises a component having an HLB value of not less than 5.0.
Thus, in a thermal transfer image-receiving sheet comprising a receptive layer provided on a substrate sheet, the incorporation of a metal source in combination with a protective layer bonding/holding agent and a binder resin into the receptive layer enables a deterioration in adhesion between the metal source-containing receptive layer and the transferred protective layer to be prevented by the protective layer bonding/holding agent, and thus can realize a thermal transfer image-receiving sheet which can provide images of a chelated dye possessing excellent quality.
Preferred embodiments of the present invention will be described in detail.
The thermal transfer image-receiving sheet according to the present invention comprises a substrate sheet and a receptive layer provided on the substrate sheet.
(Substrate Sheet)
The substrate sheet functions to hold the receptive layer, and is heated at the time of thermal transfer. Therefore, the substrate sheet preferably has mechanical strength on a level such that, even in a heated state, the substrate sheet can be handled without any trouble. Materials for such substrate sheets are not particularly limited, and examples of substrate sheets usable herein include: various types of paper, for example, capacitor paper, glassine paper, parchment paper, or paper having a high sizing degree, synthetic paper (such as polyolefin synthetic paper and polystyrene synthetic paper), cellulose fiber paper, such as wood free paper, art paper, coated paper, cast coated paper, wall paper, backing paper, synthetic resin- or emulsion-impregnated paper, synthetic rubber latex-impregnated paper, paper with synthetic resin internally added thereto, and paperboard; and films of polyester, polyacrylate, polycarbonate, polyurethane, polyimide, polyether imide, cellulose derivative, polyethylene, ethylene-vinyl acetate copolymer, polypropylene, polystyrene, acrylic resin, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyvinyl butyral, nylon, polyether ether ketone, polysulfone, polyether sulfone, tetrafluoroethylene-perfluoroalkyl vinyl ether, polyvinyl fluoride, tetrafluoroethylene-ethylene, tetrafluoroethylene-hexafluoropropylene, polychlorotrifluoroethylene, polyvinylidene fluoride and the like. Further, for example, white opaque films produced by adding a white pigment or a filler to these synthetic resins and forming films from the mixtures, or foamed sheets produced by foaming the resin may also be used without particular limitation.
A laminate of any combination of the above substrate sheets may also be used. Examples of representative laminates include a laminate composed of a cellulose fiber paper and a synthetic paper and a laminate composed of a cellulose fiber paper and a plastic film. The thickness of the substrate sheet may be any desired one, and is generally about 10 to 300 xcexcm. When the substrate sheet has poor adhesion to the receptive layer formed on its surface, the surface of the substrate sheet is preferably subjected to primer treatment or corona discharge treatment.
(Receptive Layer)
The receptive layer is provided on one side of the substrate sheet, and comprises a metal source, a protective layer bonding/holding agent, a binder resin, and optional additives such as a release agent. The binder resin is preferably easily dyeable with a sublimable dye. Binder resins usable herein include polyolefin resins, such as polypropylene, halogenated resins, such as polyvinyl chloride and polyvinylidene chloride, vinyl resins, such as polyvinyl acetate and polyacrylic esters, polyester resins, such as polyethylene terephthalate and polybutylene terephthalate, polystyrene resins, polyamide resins, resins based on copolymers of olefins, such as ethylene or propylene, with other vinyl monomers, ionomers, and cellulose derivatives. The receptive layer preferably contains a release agent from the viewpoint of preventing the heat fusion to the dye layer to prevent abnormal transfer.
The release agent is preferably a silicone oil because the silicone oil bleeds from the interior of the receptive layer onto the surface of the receptive layer to easily form a release layer on the surface of the receptive layer. Preferred silicone oils include phenyl-modified, carbinol-modified, amino-modified, alkyl-modified, epoxy-modified, carboxyl-modified, alcohol-modified, fluorine-modified, and other modified silicone oils.
Particularly preferred are modified silicone oils represented by the following chemical formula which do not adversely affect the metal source and the protective layer bonding/holding agent, do not adversely affect a chelating reaction of the dye, which can be chelated, from the dye layer with the metal source, and have excellent releasability from the dye layer. 
wherein A represents an aryl group, such as a phenyl group; B represents an epoxy-modified alkyl chain; and l and m are an integer of 1 or more.
It is also possible to use a reaction cured product of a plurality of modified silicone oils, such as a product of a reaction of a vinyl-modified silicone oil with a hydrogen-modified silicone oil or a cured product of a reaction of an amino-modified silicone oil with an epoxy-modified silicone oil, and a reaction cured product prepared by reacting an active hydrogen-containing modified silicone oil with a curing agent reactive with active hydrogen.
The amount of the release agent added is preferably 0.5 to 10% by mass based on the solid content of the receptive layer.
In the thermal transfer image-receiving sheet according to the present invention, the metal source is preferably added in an amount of not more than 50% by mass, particularly preferably not more than 40% by mass, based on the binder resin in the receptive layer, for example, because, when the metal source is mixed with the protective layer bonding/holding agent, excellent adhesion can be provided between the receptive layer and the protective layer.
According to the present invention, preferred metal sources are compounds represented by formula (I):
M2+(X)n2Yxe2x88x92xe2x80x83xe2x80x83(1) 
wherein M2+ represents a divalent transition metal ion; X represents a coordination compound which can be coordinately bonded to the transition metal ion M2+ to form a complex; n is an integer of 2 or 3; and Yxe2x88x92 represents a counter ion of the transition metal ion M2+, provided that a plurality of coordination compounds Xs may be the same or different.
In the compounds represented by formula (1), M2+ represents a divalent transition metal ion, and examples of transition metal ions include cobalt(2+), nickel(2+), copper(2+), zinc(2+), and iron(2+). Among them, nickel(2+), copper(2+), and zinc(2+) are particularly preferred. In the compounds represented by formula (1), (X)n represents two or three coordination compounds which can coordinately bond to transition metals to form complexes. The coordination compound can be selected from coordination compounds described, for example, in xe2x80x9cKireto Kagaku (Chelate Chemistry) (5)xe2x80x9d (edited by Nan""un-do Co., Ltd.). Among them, ethylenediamine derivatives, picolinamide derivatives, 2-aminomethylpiperidine derivatives, and glycinamide derivatives are preferred. Particularly preferred are ethylenediamine derivatives and glycinamide derivatives.
In the compound represented by formula (1), Yxe2x88x92 represents a counter anion of the transition metal ion M2+. This counter anion is an organic or inorganic anion, and, in particular, is preferably a compound which can render the complex of the transition metal ion M2+ with the coordination compound (X)n dissolvable in an organic solvent, for example, methyl ethyl ketone or tetrahydrofuran (THF). Specific examples of counter anions include organic salts of alkylcarboxylic acids, arylcarboxylic acids, alkylsulfonic acids, arylsulfonic acids, alkylphosphoric acids, arylphosphoric acids, and arylboric acids. Among them, for example, organic salts of arylboric acids and arylsulfonic acids are particularly preferred.
The receptive layer according to the present invention preferably contains a metal source represented by formula (2):
M2+(Xxe2x88x92)2xe2x80x83xe2x80x83(2) 
wherein M2+ represents a divalent transition metal ion; and Xxe2x88x92 represents a coordination compound represented by formula (1). The compound represented by formula (2) may have a neutral ligand according to a central metal, and representative ligands include H2O and NH3.
Metal sources represented by formula (2) include those wherein X represents a coordination compound represented by formula (3): 
In the compound represented by formula (3), Z represents an alkyl, aryl, alkoxy, acyl, alkoxycarbonyl, aryloxycarbonyl, or carbamoyl group or a halogen or hydrogen atom. Preferred Zs are electron-withdrawing groups, such as aryloxycarbonyl groups, alkoxycarbonyl groups, and halogen atoms, because they can stabilize metal ion donating compounds. Among them, aryloxycarbonyl groups and alkoxycarbonyl groups are further preferred from the viewpoint of solubility. Aryloxycarbonyl groups include a phenoxycarbonyl group, and alkoxycarbonyl groups include straight-chain or branched alkoxycarbonyl groups having 1 to 20 carbon atoms, such as methoxycarbonyl, ethoxycarbonyl, pentyloxycarbonyl, and 2-ethylhexyloxycarbonyl groups. These alkoxycarbonyl groups may be substituted, for example, by a halogen atom, an aryl group, or an alkoxy group.
R and Rxe2x80x2, which may be the same or different, each represent an alkyl or aryl group. R and Z or Rxe2x80x2 and Z may combine with each other to form a ring, provided that when Z represents a hydrogen atom, both R and Rxe2x80x2 do not simultaneously represent a methyl group. Alkyl groups represented by Z, R, and Rxe2x80x2 include, for example, straight-chain or branched alkyl groups having 1 to 20 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, t-butyl, hexyl, octyl, and 2-ethylhexyl groups. These alkyl groups may be substituted, for example, by a halogen atom, an aryl group, or an alkoxy group. Aryl groups represented by Z, R, and Rxe2x80x2 include substituted or unsubstituted phenyl and naphthyl groups. Alkoxy groups represented by Z include straight-chain or branched alkoxy groups having 1 to 20 carbon atoms, such as methoxy, ethoxy, and butoxy groups. Acyl groups represented by Z include acetyl, propionyl, chloroacetyl, phenacetyl, and benzoyl groups. The halogen atom represented by Z is preferably a chlorine atom.
The amount of the metal source used in the present invention is preferably 20 to 50% by mass, more preferably 30 to 40% by mass, based on the binder resin for a receptive layer. The metal source according to the present invention is not limited to the compounds represented by formulae (1) and (2).
The protective layer bonding/holding agent used in the thermal transfer image-receiving sheet according to the present invention is preferably a fluorosurfactant, and examples thereof include perfluoroalkylsulfonamide esters of polyethylene oxide (FC-430, FC-431, and FC-170, manufactured by Sumitomo 3M Ltd.; and EF-122A, EF-122B, EF-122C, EF-122A3, and EF-501, manufactured by Mitsubishi Materials Corporation.).
Additional examples of the protective layer bonding/holding agent include perfluoroalkyl ethylene oxide adducts (F-142D, F-144D, and F-1405, manufactured by Dainippon Ink and Chemicals, Inc.; KH-40, manufactured by SEIMI CHEMICAL CO., LTD.; and DS-401 and DS-403, manufactured by Daikin Industries, Ltd.).
Further examples of the protective layer bonding/holding agent include hydrocarbon acrylate-perfluorocarbon acrylate copolymers (EF-351, EF-352, EF-801, EF-802, and EF-6011, manufactured by Mitsubishi Materials Corporation.).
Still further examples of the protective layer bonding/holding agent include fluoroalkyl polyoxyethylene ethers (FTX-251 and FTX-22, manufactured by Neos Co., Ltd.).
Preferred polyether-modified silicones include those which have been modified by providing a polyoxyalkylene group on its side chain and have an HLB value (hydrophile-lipophile balance), which is an aspect representing the properties of surfactants, of not less than 5.0.  HLB  =                    Molecular weight of hydrophilic group                    Molecular weight of surfactant              xc3x97          100      5      
Reference: Awa No Hassei Mekanizumu To Seigyo Oyobi Toraburu Taisaku (Mechanism and control of foam formation and measures against troubles), published by Gijutu Joho Kyokai.
The protective layer bonding/holding agent is preferably added in an amount of 0.25 to 7.5% by mass based on the solid content of the whole receptive layer. It is considered that, while the metal ion-containing compound and the release agent present in the receptive layer inhibit the adhesion of the receptive layer to the protective layer, the protective layer bonding/holding agent covers the metal ion-containing compound and the release agent on the surface of the receptive layer and, in addition, can improve the wettability to enhance the adhesion between the receptive layer and the transferred protective layer. When the content of the protective layer bonding/holding agent is less than 0.25% by mass based on the solid content of the whole receptive layer, the adhesion between the receptive layer and the transferred protective layer is disadvantageously deteriorated. On the other hand, when the content of the protective layer bonding/holding agent exceeds 7.5% by mass based on the solid content of the whole receptive layer, the wettability of the surface of the receptive layer is so high that the relesability between the receptive layer and the dye layer is disadvantageously deteriorated.
The receptive layer may be formed by coating an ink, prepared by adding a metal source, a protective layer bonding/holding agent, and optional additives, such as a release agent, to a binder resin, dissolving or dispersing the mixture in water or a solvent such as an organic solvent, onto a substrate sheet by a conventional method, such as bar coating, gravure printing, screen printing, or reverse roll coating using a gravure plate and drying the coating. Instead of this method wherein an ink is coated directly onto a substrate sheet to form a coating which is then dried, a method may be adopted wherein a receptive layer may be transferred onto a substrate sheet from a receptive layer transfer sheet comprising a receptive layer provided on a different substrate sheet. This different substrate sheet may be formed of the same material as used in the substrate sheet. The thickness of the receptive layer is preferably about 0.1 to 10 xcexcm on a dry basis, i.e., after coating and drying.
The thermal transfer sheet used in the formation of an image on the thermal transfer image-receiving sheet comprises a substrate and, provided on the substrate, a dye layer composed mainly of a thermally transferable dye, which can be chelated, and a binder resin. The thermally transferable dye is not particularly limited, and conventional thermally transferable dyes may be used so far as the dye has a group which can combine with the above-described metal source to form a complex.
The protective layer transfer sheet for use in the formation of a protective layer by thermal transfer onto the image forming face (receptive layer face) in the thermal transfer image-receiving sheet according to the present invention will be described.
(Substrate Sheet)
The same substrate sheet as used in the conventional thermal transfer sheet as such may be used as the substrate sheet used in the protective layer transfer sheet according to the present invention. Further, a film, the surface of which has been subjected to easy-adhesion treatment, and other substrates may also be used without particular limitation. The thickness of the substrate sheet may properly vary depending upon the material constituting the substrate so that the strength and heat resistance of the substrate sheet are proper. In general, the thickness is preferably about 3 to 100 xcexcm.
(Release Layer)
In general, a thermally transferable resin layer is provided on one side of the substrate sheet to prepare a protective layer transfer sheet. In some cases, for some combination of the material constituting the substrate sheet with the material constituting the thermally transferable resin layer, the releasability is not good at the time of the thermal transfer. In this case, a release layer may be previously provided on the surface of the substrate sheet.
The release layer may be formed by coating a coating liquid containing at least one of a wax, a silicone wax, a silicone resin, a fluororesin, an acrylic resin, polyvinyl alcohol, a cellulose derivative resin, a urethane resin, an alkyl vinyl ether/maleic anhydride copolymer resin and the like by a conventional method such as gravure coating or gravure reverse coating and drying the coating. A coating thickness of about 0.1 to 2 xcexcm suffices for the release layer. What is important for selecting the material used in the release layer is, of course, that the material has proper releasability from the thermally transferable resin layer. Further, it is important that the adhesion of the material to the substrate sheet be higher than the adhesion of the material to the thermally transferable resin layer. Unsatisfactory adhesion of the material to the substrate sheet is causative of abnormal transfer, for example, such that the release layer, together with the transferable resin layer, is transferred.
When a matte protective layer is desired on an as-transferred print, a method may be used wherein various particles are incorporated into the release layer, or wherein a substrate sheet in its surface, on the release layer side, which has been matted, may be used to render the surface of a print, with the protective layer transferred thereon, matte.
(Thermally Transferable Resin Layer)
In the protective layer transfer sheet, the thermally transferable resin layer is thermally transferred onto an object in its print face to form a protective layer. Therefore, functions, which the thermally transferable resin layer should have, include sure, i.e., good, separation and transfer from the substrate sheet or the release layer provided on the substrate sheet at the time of the thermal transfer, thermal adhesion to an object, a capability of imparting, as a protective layer for the print face, various resistance properties, such as abrasion resistance and scratch resistance, and transparency high enough not to deteriorate the sharpness of the transferred image underlying the protective layer. Therefore, the thermally transferable resin layer may have a single-layer structure. Alternatively, a thermally transferable resin layer having a two-layer, three-layer, or other multi-layer construction, for example, comprising a transparent resin layer and thermally adhesive resin layer provided in that order from the substrate sheet side, or a transparent resin layer, an ultraviolet cut-off layer, and a thermally adhesive resin layer provided in that order from the substrate sheet side is also preferred. Each layer used in this case will be described.
(Transparent Resin Layer)
The transferable resin layer provided on the substrate sheet or the release layer, that is, the layer, of the thermally transferable resin layer, on the substrate sheet side, may be formed of, for example, a resin possessing, for example, excellent abrasion resistance, transparency, and hardness, for example, polyester resin, polystyrene resin, acrylic resin, polyurethane resin, acrylated urethane resin, a silicone modification product of these resins, a mixture of these resins, or a resin prepared by crosslinking and curing at least one of a polymerizable monomer, an oligomer, a reactive polymer described below by exposure to an ionizing radiation such as an electron beam. The cured resin layer may contain, as a mixture thereof, a highly compatible thermoplastic resin from the viewpoint of improving flexibility and adhesion.
Although these resins have excellent transparency, they are likely to form a relatively strong film which has unsatisfactory tranferability at the time of thermal transfer. In order to improve, for example, the transferability of the transparent resin layer, abrasion resistance and scratch resistance of the print face on which the resin is covered by transfer, for example, highly transparent fine particles, such as silica, alumina, calcium carbonate, plastic pigments, or waxes may be added in such an amount that does not sacrifice the transparency of the resin. The particle diameter of the fine particles is preferably about 0.1 to 10 xcexcm. Further, silicone-modified resins, lubricants or other additives may be added to further improve the abrasion resistance and the scratch resistance.
Gravure coating, gravure reverse coating, roll coating, and other various means may be utilized for the formation of the transparent resin layer. The transparent resin layer may be formed by coating a coating liquid containing the above resin by the above means and drying the coating. The thickness of the transparent resin layer is about 0.1 to 50 xcexcm, preferably about 1 to 10 xcexcm, on a dry basis.
(Ultraviolet Cut-Off Layer)
An ultraviolet cut-off layer formed of a thermally transferable resin with an ultraviolet absorber added thereto is preferably provided from the viewpoint of preventing images formed on a print, onto which the protective layer is to be transferred, from fading or discoloring upon exposure to ultraviolet light contained, for example, in sunlight. Ultraviolet absorbers usable herein include conventional organic ultraviolet absorbers, such as benzophenone compounds, benzotriazole compounds, oxalic anilide compounds, cyanoacrylate compounds, and salicylate compounds. Further, inorganic fine particles having ultraviolet absorbing activity, such as oxides of zinc, titanium, cerium, tin, and iron may be incorporated into the resin layer. The resin used is not particularly limited, and any resin may be used. Examples of resins usable herein include hydrocarbon resins, such as acrylic resins, polyester resins, urethane resins, styrene resins, halogenated vinyl resins, vinyl acetate resins, polycarbonate resins, phenolic resins, melamine resins, epoxy resins, cellulosic resins, and polyethylene resins, vinyl resins, such as polyvinyl alcohol and polyvinyl pyrrolidine, and copolymers of monomers constituting the above resins. Further, the ultraviolet absorber may be incorporated into the transparent resin layer without specially providing the ultraviolet cut-off layer.
Alternatively, a resin formed by bonding through a reaction a reactive ultraviolet absorber to at least one of a monomer, oligomer, and reactive polymer of a thermoplastic resin is incorporated, solely or as a mixture of two or more types, into any layer constituting the transparent resin layer. The reactive ultraviolet absorber may be fixed through a reaction to the resin by various methods, and an example thereof is to radically polymerize a resin component of a conventional monomer, oligomer, or reactive polymer and the following reactive ultraviolet absorber having an addition polymerizable double bond to prepare a copolymer.
(Thermally Adhesive Resin Layer)
The thermally adhesive resin layer functions to facilitate the transfer of the thermally transferable resin layer onto an object and, at the same time, to enhance the adhesion of the thermally transferred resin layer to the object. This thermally adhesive resin layer may be formed of a heat-melt adhesive, such as acrylic resin, styrene-acryl copolymer, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, polyester resin, or polyamide resin. The thermally adhesive resin layer may be formed by a conventional method such as gravure coating, gravure reverse coating, or roll coating. The thickness of this layer is preferably about 0.1 to 5 xcexcm on a dry basis.
Thermal transfer recording is performed, on the above-described thermal transfer image-receiving sheet, using a thermal transfer sheet comprising a substrate and, provided on the substrate, a dye layer, composed mainly of a thermally transferable dye, which can be chelated, and a binder resin. In this case, thermal energy in response to image information is applied by conventional thermal energy application means, such as a thermal head, a laser beam, an infrared flash lamp, or a hot pen.
Further, after thermal transfer recording on the thermal transfer image-receiving sheet according to the present invention to form an image, a protective layer (a thermally transferable resin layer) is thermally transferred from a protective layer transfer sheet onto the image to prepare a print with a protective layer formed thereon. In this case, heating means for the transfer of the protective layer is not limited to a thermal head, and other means, such as hot plates, hot stampers, hot rolls, line heaters, or irons, may be used. The protective layer may be transferred onto the whole area of the receptive layer face including the formed image, or alternatively may be transferred onto a specific portion of the receptive layer face.